The successful treatment of glaucoma hinges upon early detection of optic nerve damage. The first reliable signs of such damage traditionally have been localized visual field defects. However, recent studies have shown that conventional visual field tests may appear normal in spite of substantial losses of retinal ganglion cells. More sensitive alternative tests are therefore needed to detect glaucomatous changes before significant fiber loss occurs. One promising candidate for a more sensitive glaucoma test is the measurement of visual fields at scotopic adaptation levels. Preliminary studies have shown that scotopic visual field defects are usually more pronounced than the comparable photopic defects, suggesting that scotopic tests may be able to detect earlier optic nerve damage. A thorough investigation of rod-mediated vision in glaucoma is proposed. Spatial, temporal and adaptation parameters will be varied to find conditions that are maximally sensitive to scotopic visual field defects in glaucoma. Scotopic visual fields will be quantitatively compared with photopic fields, retinal nerve fiber layer photographs, and stereoscopic optic disc photographs from the same patients. The proposed studies are directed toward the following goals: (1) confirm that scotopic visual field defects are more severe than photopic defects in glaucoma and quantify the photopic-scotopic difference for various amounts of field loss; (2) test the hypothesis that scotopic visual field defects can be detected earlier than comparable photopic defects; (3) develop clinical scotopic perimetry tests that are sensitive to early optic nerve damage; (4) correlate the psychophysical findings with other evidence for selective ganglion cell damage in glaucoma.